pidfs: remove pidfs_{get,put}_pid()

pidfs: remove pidfs_{get,put}_pid()

Now that we stash persistent information in struct pid there's no need
to play volatile games with pinning struct pid via dentries in pidfs.

Link: https://lore.kernel.org/20250618-work-pidfs-persistent-v2-8-98f3456fd552@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>